The present invention relates generally to plastic molding and more particularly to a process and apparatus for forming molded plastic articles, such as automotive parts, dashboards and the like.
In slush casting of thin-walled articles from castable plastic material, such as plastisol and pourable plastic powders, it is known to preheat a mold in a hot air furnace to a temperature above the optimum molding temperature, to remove the mold from the furnace, and to allow the mold temperature to drop to the desired optimum molding temperature. According to known practice, the heated mold is then filled with liquid plastic, such as plastisol or pourable plastic powder, the mold is rotated and the plastic is allowed to set up and cure in the heated mold.
According to this known practice, the desired mold temperature is determined by overheating the mold and then allowing the mold to cool for a predetermined time, selected based upon a previously measured or known time-temperature curve for the mold being used. Hence, achieving the correct mold temperature is a matter of controlling the time after the mold is removed from the heating oven until the plastic material is added. The accuracy of such a technique in achieving the correct mold temperature thus depends upon the accuracy of the time-temperature curves for the particular mold. In practice, different molds may have subtle or even substantial differences in time-temperature characteristics, so that the optimum mold temperature is at best an approximation. Deviations from the optimum mold temperature can result in lack of uniformity in the molded articles and high scrap rates or reject rates.
In addition to poor mold temperature control, hot air furnaces used in the prior art are energy inefficient and expensive to operate. Each time the furnace or oven is opened to insert or remove a mold, much energy is lost as waste heat. This waste heat places an additional burden on the manufacturing facility's air conditioning equipment and the cost of this waste heat is reflected in a higher per unit cost for manufacturing the plastic article.
The present invention overcomes the above deficiencies by providing a process and apparatus for forming molded plastic articles in which the mold is heated by contact with a heated salt bath. The mold temperature is monitored by a noncontact sensor while the mold is in the salt bath. The mold is removed from the salt bath and a plastic material is added as soon as the optimum mold temperture is reached. The invention thus places no critical reliance on time-temperature curves, which can vary from mold to mold. In addition, the salt bath is self-insulating; hence, very little waste heat is lost to the environment.
According to the invention, a process for forming a molded plastic article comprises providing a mold having a cavity and an external surface, and also providing a salt bath at a first predeterminded temperature. The cavity is heated by contacting the external surface of the mold with the salt bath, the mold is then removed from the salt bath and a plastic material is placed in the heated cavity and is allowed to cure into the plastic article. The plastic article is then removed from the cavity.
The invention also provides a method for forming a molded plastic article comprising providing a mold having a cavity and exposing the mold to a heat source. While the mold is exposed to the heat source, the mold temperature is sensed and the mold is removed from the heat source when a predetermined mold temperature is reached. A plastic material is then placed in the cavity and allowed to cure into the plastic article. The plastic article is then removed from the cavity.
The invention is well adapted for automated assembly line applications. Thus, the invention also provides an apparatus for forming a molded plastic article comprising a salt bath heating station having a means associated with the heating station for sensing mold temperature. The apparatus provides at least one mold having an internal cavity and an external surface. A charging station is also provided, including a container for holding a supply of plastic material, the container having an opening therein through which a charge of plastic material may be introduced into the mold cavity.
The invention further comprises a means for placing the external surface of the mold in contact with the salt bath, to thereby heat the internal cavity. A means for conveying the mold from the heating station to the charging station is also provided.
The invention further comprises a means for placing the mold in registration with the container opening, with the cavity facing into the opening. The invention provides a means for simultaneously rotating the mold and the container about a common axis to thereby introduce a charge of plastic material into the cavity.
For a more complete understanding of the invention, its objects and advantages, reference may be had to the accompanying drawings and to the following specification.